In humans, angioplasty is an increasingly common invasive vascular procedure whereby luminal narrowing due to increases in intima thickness is reduced by mechanical dilation. One common form of this procedure is where a balloon catheter is expanded at a narrowed site in a blood vessel. Unfortunately, angioplasty often also results in vascular injury which produces subsequent narrowing at the site of angioplasty, referred to as restenosis, as a direct response. The exact causes of restenosis have yet to be completely elucidated, but have been postulated to involve arterial wall remodeling and/or intimal hyperplasia (IH). IH is characterized by cellular proliferation and the accumulation of matrix components on the inner wall of blood vessels, leading to luminal narrowing. IH is also characterized by a thickened fibromuscular layer between the blood vessel's endothelium and the inner elastic lamina (IEL).
Considerable research has aimed at clinical and pharmacological intervention to prevent or treat restenosis, including experimental attempts to use photodynamic therapy (PDT) in animal models. Generally, PDT involves the use of an inert photosensitizer (PS) that is activated by specific wavelength(s) of radiation to produce a toxic agent, thought to be singlet oxygen in the case of porphyrins, which causes the destruction of unwanted cellular tissue. The PS is essentially a catalyst for generating the toxic agent.
Some research has been based upon the observation that PDT may be adapted for photoangioplasty (see Rockson et al. Circulation 102:591-596, 2000) for a recent review. Other research has been based upon the use of PDT to kill smooth muscle cells (SMCs) in vitro (see Sobeh et al. Eur. J. Vasc. Endovasc. Surg. 9:463-468, 1995).
Research with animal models to study the possible use of PDT to prevent or treat human restenosis utilize normal arteries. This is despite the contrasting situation with restenosis in human arteries, which is the response to angioplastic treatment of arteries with pre-existing disease. Such arteries contain advanced atherosclerotic lesions, often with regions of calcification. Treatment of such diseased arteries with angioplastic procedures, such as percutaneous transluminal coronary angioplasty (used as an alternative to coronary artery bypass grafting), frequently results in considerable tissue damage, including tears in the intima and media of the artery. This degree of injury, and thus the subsequent restenotic response, is inadequately modeled in most animal systems used, where simple endothelial denudation, rather than the deeper injury seen with human angioplasty, is used. As such, the results with animal models should be reviewed with caution. For reviews of animal models and restenosis, see Lafont et al. (Cardiovascular Res. 39:50-59, 1998), Ferns et al. (Int. J. Exp. Path. 81:63-88, 2000), and Schwartz (Lab. Investig. 71(6):789, 1994). Interestingly, some of this work has led to the suggestion that arterial remodeling, rather than intimal formation, accounts for restenosis after angioplasty (see Lafont et al.)
One example of PDT in an animal model is described by Asahara et al. (Circulation 86 (Suppl) 1-846, 1992). They observed that PDT was able to inhibit restenosis in rabbits that had been fed a high cholesterol diet and received balloon injuries of the iliac artery. Importantly, they found that immediate PDT treatment following balloon injury was ineffectual at decreasing intimal thickness (expressed as the intima:media, I/M, ratio). Instead, they observed the best results by using PDT one week after balloon injury. Gonschior et al. (Photochemistry and Photobiology, 64(5):758-763, 1996) conducted similar PDT experiments in a swine model using injury to nondiseased arteries (see also Gonschior et al., J. Amer. Coll. Cardiol. 27(2, Suppl A):196A, 1996).
Other workers have used PDT to inhibit IH and completely deplete endothelial cells, as well as medial cells in some instances, from rat arteries (Nyamekye et al. Eur. J. Vasc. Endovasc. Surg. 11: 19-28, 1996; LaMuraglia et al. Photodynamic therapy inhibition of experimental intimal hyperplasia: acute and chronic effects. J Vasc Surg, 19:321-331, 1994; Grant et al. Br. J. Cancer 70:72-78, 1994; and Ortu, P. et al. Circulation 85:1189-1196, 1992). In all of these instances, the photosensitizer was administered systemically and the radiation performed external to the blood vessel. For example, Ortu et al. used endothelial denudation (with an embolectomy catheter) of normal rat carotid arteries to study PDT with systemically administered chloraluminum-sulfonated phthalocyanine (CASPc) used as the photosensitizing agent. They noted that their denudation protocol did not disrupt the internal elastic lamina (EL) and was effective in inhibiting the induced IH response on day 14 when PDT was conducted on days 2 and 7 after denudation. External irradiation of the artery was used to activate CASPc. It is unknown whether complete depletion as seen in these methods would be safe or advantageous in humans.
Similar depletion of endothelial cells and medial cells were observed by Adili et al. (Lasers in Surgery and Medicine 23:263-273, 1998; and SPIE 2395:402-408, 1995) upon the use of PDT with locally administered photosensitizer in injured rat carotid arteries and 100 J/cm2 external photoactivation. In the 1998 work, they reported that the photosensitizer dosage in the vessel wall was approximately 2.5 ng/mg after local pressurized delivery of a 25 μg/ml solution for two minutes and approximately 1.4 ng/mg after systemic delivery of a 2 mg/ml solution via injection. In the 1995 work, they reported similar concentrations of approximately 1.5 ng/mg with both local and systemic administration, at 25 μg/ml and 0.5 mg/kg body weight, respectively. 15 minutes after localized delivery, irradiation was performed externally with 100 J/cm2 of 690 nm light.
Eton, D. et al. (J. Surg. Res. 53:558-562, 1992) used a similar endothelial denudation (with a balloon catheter) of normal rabbit carotid arteries to study PDT with systemically administered Photofrin™. This denudation protocol again leaves the media and adventitia of the artery intact, and PDT was effective in reducing the induced IH response (recorded as a ratio of IH area to normalized area enclosed by the IEL based on cross sectional analysis of the arteries five weeks after PDT). Photofrin™ was systemically administered 7 days after denudation, with external irradiation of the artery two days later. In a similar model, Hsiang et al. (Ann. Vasc. Surg. 9:80-86, 1995) reported the prevention of IH with PDT in rabbit arteries injured by endothelial denudation. In this approach, Photofrin™ was administered systemically after injury, with 120 or 240 J/cm2 irradiation following 24 hours later.
In a porcine model for the prevention of IH, however, Sobeh et al. (SPIE 2395:390-395, 1995) observed the failure of systemic administration of photosensitizer followed by balloon injury and external irradiation to result in the prevention of IH. Instead, they observed that PDT under such conditions resulted in a significant increase of intimal area. The PDT conditions were injection of 0.3 mg/kg of metatetrahydroxyphenyl-chlorin (m-THPC) four hours prior to endothelial denudation and external irradiation with 20 J/cm2 of 652 nm light.
One possible explanation for the above results may be based on the observations of Adili et al. (Photochemistry and Photobiology, 70(4):663-668, 1999) concerning the applied dose of PDT. With the rat carotid artery model, Adili et al. used balloon mediated endothelial denudation before pressurized local delivery of either a 0.5 μg/ml or a 25 μg/ml solution of BPD-MA for two minutes followed 15 minutes later by external irradiation with either 50 or 100 J/cm2 light of 690 nm. The use of 0.5 μg/ml BPD-MA with 50 and 100 J/cm2 light (Stages I and II, respectively) was observed to have little effect on reducing IH. Instead Stage II is noted to be associated with induction of significant IH (see page 666, top of right column). The use of 25 μg/ml BPD-MA with 50 and 100 J/cm2 light (Stages III and IV, respectively) was observed to inhibit IH and induce photothrombosis, respectively. As such, Adili et al. suggest that correct dosimetry is needed to effectively inhibit IH. In particular, relatively low dose PDT is ineffectual while relatively high doses may trigger thrombosis. See Hsiang et al. (Cardiovasc. Surg. 3(5):489-493, 1995, and Photochem. Photobiol. 53(3):518-525, 1993) for previous studies on PS dosages.
Lastly, a variation of PDT, wherein no photoactivation is induced, has also been described as inhibiting restenosis and intimal hyperplasia (see U.S. Pat. No. 5,422,362, which is hereby incorporated by reference in its entirety as if fully set forth).